Measurement of the muon momentum in pion decay at rest using a surface muon beam

Abstract

We have measured the momentum of muons from the decay π + → μ +ν μ at rest to be p μ + = 29.79207±0.00012 MeV/c. This result leads to a laboratory upper limit of 0.16 MeV ( CL = 0.9) for the muon-neutrino mass. The cosmological upper limit of the neutrino mass, the muon mass and the new p μ + -value yield the pion mass m π + = 139.57031 ± 0.00016 MeV. The mean kinetic energy of the pions stopped in the isotropic-graphite production target immediately before their decay is found to be T π + = 0.415 ± 0.027 eV .